Permalinks Help

I’ve recently developed a link directory plugin for WordPress and I’m trying to enable permalinks to correspond to categories in much the same way as WordPress uses /%category%/.

I read through this guide:
https://codex.www.ads-software.com/Custom_Queries#Custom_Archives

..and then tried it myself. My code looks like this:
https://pastebin.com/m400f1341

Yet when I type in https://www.mysite.com/country/Ireland in my address bar I’m simply presented with the homepage, without any echo’d text from the script. Without the code in place that URL returns a 404 error, but with it in place it returns the homepage.

I think that the permalinks are being loaded but for some reason the $_GET variables aren’t reaching my script. What am I missing?